Many electronic devices employ a traveling stream of charged-particles, such as electrons, formed into a beam as an essential function in the device's operation. For example, one type of vacuum device, a microwave traveling wave tube, incorporates a source of electrons that are formed into a beam, in which the electrons are accelerated to a predetermined velocity and directed along an axial path through an "interaction" region within the microwave tube body. In the interaction region, kinetic energy is transferred from the moving electrons to the high frequency electromagnetic fields, such as microwave signals, that are propagating along a slow wave structure through the interaction region at about the same velocity as the moving electrons. The electrons give up energy to the microwave field through the exchange process characterized as electronic interaction, evidenced by a lower velocity of the electrons exiting from the interaction region. The "spent" electrons pass out of the interaction region where they are incident upon and collected by a final tube element, termed the collector. The collector collects and returns the incident electrons to the voltage source. As is recognized, much of the energy in a moving particle is released in the form of heat when the particle strikes a stationary element, such as the collector. This produces undesired heating in the microwave tube and a lower overall electrical efficiency of microwave tube operation.
The depressed collector and, more particularly, the multistage depressed collector is a collector that increases the electrical efficiency of traveling wave tube operation as well as reduces undesireable heat generation by a process of velocity sorting of the electrons controlled by a retarding electric field. The field slows the electrons so that the electrons are collected by electrodes at a reduced velocity and ideally at a zero velocity. As is known to those skilled in the art, the multistage depressed collector is characterized physically by a series of spaced metal electrodes, each containing a passage therethrough, a final electrode and a passage entry for receiving electrons. The electrodes are maintained at successively lower voltages with respect to the tube circuit taken as ground (or at successively higher negative voltages as otherwise viewed) so as to present a retarding electric field to the electrons which pass through the entrance into the collector region. Such types of devices are substantially well developed and hence are complex in nature as is known to the reader skilled in the art.
One type of known multistage depressed collector employs a combination of a transverse electric field and a longitudinal magnetic field for sorting electrons as a function of electron velocity. See U.S. Pat. No. 3,526,805, by Okashi, et al.; No. 3,644,778, by Mihran, et al.; and No. 3,702,951, by Kosmahl. Another type of collector employs a retarding electric field established by a cuplike electrode and a pointed spike located in the center of the cuplike member. The effect of this structure with a voltage applied is to present an electron mirror with a negative focal length to electrons moving near the axis. Hence, the reflected beam is more divergent than the incident beam. See the paper entitled Multistage Depressed Collector Investigation For Traveling Way Tubes, Tammaru, NASA CR-72950 EDDW-3207, Contract NAS-3-11536, Final Contract Report.
The efficiency of the NASA collector is limited by the defocusing properties of the spikelike reflector. Further, the collectors shown in the patents mentioned above required the maintenance of an axial magnetic field of a critical magnitude for proper functioning.
An improved multistage depressed collector utilizing electrodes that are asymmetrical was disclosed in U.S. Pat. No. 4,096,409, by Hechtel, which is assigned to the same assignee as the present invention. This electron collector has a high efficiency and utilizes the concept of focusing electrons to collection points on various electrodes depending upon the energy level of the electron. One disadvantage of the Hechtel patent is that it is difficult to fabricate and align the various asymmetric electrodes.